This invention relates to a method and a device for controlling the thickness of a sheet to be a target thickness during forming it by calender rolls.
In general, when a sheet is being formed by calender rolls, the thickness of the sheet often changes owing to change in physical properties of materials of the sheet, forming temperatures and the like. In order to overcome this problem, it has been proposed that the thicknesses of a sheet is measured immediately after it has been formed to obtain a difference between the measured value and a target value. At least one of the calender rolls is then moved correspondingly to the obtained difference to adjust the gap between the calender rolls, thereby controlling the thickness of the sheet to be the target thickness.
The principle of such a control of the thickness will be explained with reference to FIG. 1 whose ordinate indicates rolling forces N of calender rolls and the abscissa shows gaps G between the calender rolls. Curves A and B in FIG. 1 indicate the rolling force characteristics of materials, which are called as "plastic curve of material in rolling". These curves progressively lower to the right in the graph, because the narrower the gap between the rolls, the higher is the rolling force. The plastic curve of material depends upon kinds of materials to be formed, working temperatures, calendering speeds, and amounts of bank materials (amounts of materials arranged between calender rolls) and moves toward the right in the graph with harder materials, lower working temperatures and more bank materials.
Straight lines a and b in the graph indicate the relationship between the rolling force N and the gap between calender rolls and rise to the right in the graph, because the calender rolls are more deformed and the frame of the calender supporting the calender rolls is more elongated as the rolling force N increases. In this case, the gradients K of the straight lines a and b are called as "mill modulas" which shows a rigidity of the calender as a whole.
It is no assumed that the calendering is performed along the curve A and the straight line a. The gap G between the calender rolls and the rolling force N are determined by the point of intersection T0 of these lines A and a, the former h0 and the latter PO. It is further assumed that the gap h0 gives a target thickness of a sheet and a material is supplied between the calender rolls with the gap h0. If the plastic characteristics of the material change to move from the curve A to the curve B, the point of intersection moves from T0 to T1 so that the gap G between the calender rolls increases by .DELTA.h to be h1.
In order to restore the thickness of the sheet to the target thickness, the increased gap h1 must be returned to h0. For this end, the roll gap control device of the calender is adjusted by an adjustment .DELTA.s to move the straight line a to the line b so that the calendering is performed along the curve B and the straight line b. Consequently, the gap G between the calender rolls becomes h0 given by the point of intersection T2 of the curve B and the straight line b. The thickness of the sheet is thus controlled to be the target thickness in this manner.
The following relationship is established between the .DELTA.s and .DELTA.h. EQU .DELTA.s=(1+M/K).times..DELTA.h
Upon substituting (1+M/K) in the equation by control coefficient .alpha., the equation reads as follows. EQU .DELTA.s=.alpha..times..DELTA.h
where M is gradient of plastic curve of material which is a gradient at the point of intersection T0, T1 or the like of the curve A or B, and K is the mill modulas described above. The above equation means that if it is required to adjust the gap G by .DELTA.h, the roll gap control device of the calender may be adjusted by .alpha..times..DELTA.h.
In the prior art, the control of the gap between calender rolls has been effected with an estimation that the gap between calender rolls will be changed some amount by adjusting the roll gap control device of a calender a certain amount in the manner described above. In such an operation, the control of the thickness of a sheet has been performed by using a constant control coefficient experimentally determined, notwithstanding that it will change when the plastic characteristics of a material to be calendered is changed. Consequently, the actually corrected amounts of the gap often deviate from the estimated amounts to an impermissible extent.
Moreover, the control coefficient includes some errors because it was experimentally determined. As a result, the actually corrected amounts of the gap tend to deviate somewhat from the estimated amounts. In order to overcome this problem, it has been proposed to correct the control coefficient .alpha. to an optimum value spontaneously in forming a sheet as disclosed in, for example, Japanese Patent Application Publication No. 62-17,527. However, this method suffers disadvantages from the very complicated control and the complicated and expensive apparatus.